The print quality which can be provided by printing machines, more particularly multi-color rotary presses, depends largely upon the ability of the consecutive gripper systems to retain the sheet reliably--i.e., to prevent the sheet from slipping--and to ensure that the original sheet register position is retained while the sheet is passing through the press. The use of art papers and viscous inks imposes stringent requirements on the means which ensure that the register is maintained. The retaining forces can be increased, but not ad infinitum, or the adhesion of the gripper means can be enhanced by surface patterning.
It is known from U.S. Pat. No. 2,933,040 for the sheet to be engaged not by the entire gripper area, but by discrete dots or tips produced by the formation of a non-etchable dot structure on the base member of a gripper support made of a hardened material and subsequent etching-out of unmasked surfaces with an acid.
A disadvantage in the foregoing disclosure is that the dot structure is produced photochemically--i.e., by etching operations in association with previous photographic operations. The resulting structure is relatively dense, containing approximately 300-400 dots per square centimeter. A surface of this kind cannot provide a satisfactory positive engagement. Also, there is a risk that the resulting fissured hooking structure may clog up rapidly with paper dust. Consequently, although a large number of paper-supporting tips or dots are provided, the effectiveness thereof when they are pressed on to the paper stock surface is considerably impaired, with the result of reduced retaining forces between the sheet gripper and the associated sheet gripper support.